why does digital volume control cause loss in info

VERY HELPFUL! Thanks Al (and everyone else too).

--dan

The problem with digital volume control is that not all algorithms properly address quantization errors

You need to apply dither in order to randomize quantization errors.

Chances are that the CD that you buy has had dither applied. Now if you modify the bits digitally and re-quantize it (at a new lower digital volume level) then you need to dither it before hand in order to avoid introducing non-random quantization errors.

iTunes applies dither in the recent versions - so you can be confident that the volume control is as good as analog. In older versions it was poor - it just threw away bits.

There is a common digital volume control chip, used in my McCormack MAP1 six channel preamp, which does 1/2 dB steps. It is a stepped attenuator, and no motor is involved. If such a chip follows the D/A conversion no bits are lost. If an analog output is being produced there is no reason to use another implementation.

Now, if you want to have a digital output and attenuate the signal there will be a loss of resolution. No matter how you process the data a 12 bit output obviously has less resolution than a 16 bit output.

Shadorne: Thanks. Not sure, though, that it's fair to say that properly applied dither will necessarily make a digital volume control "as good as analog." It will certainly reduce the subjective objectionability of quantization noise. But I suspect that it could not adequately compensate for the loss of say 4 or 5 bits, corresponding to a volume control setting that is 24 or 30 db below maximum.

Also, it would seem expectable that in a standalone audio component, that most likely does not have the computing horsepower of a general purpose computer, and that has to calculate and apply the digital volume control function and the corresponding dither on-the-fly, in real time, for any arbitrary volume setting, that optimal dithering and noise shaping is not necessarily going to happen.

ElDartford: If an analog output is being produced there is no reason to use another implementation.

Cost, perhaps? In other words, reducing cost by eliminating that chip altogether, and incorporating the digital volume control function within a gate array or other custom digital chip that is required for other purposes.

Regards,
-- Al

Shadorne: Thanks. Not sure, though, that it's fair to say that properly applied dither will necessarily make a digital volume control "as good as analog." It will certainly reduce the subjective objectionability of quantization noise. But I suspect that it could not adequately compensate for the loss of say 4 or 5 bits, corresponding to a volume control setting that is 24 or 30 db below maximum.

You have to consider that most home audio systems have a dynamic range of only about 60 to 70 db to begin with.

Think about a typical noise floor of around 30 db SPL and then a typical max undistorted SPL of 100 db SPL (played at maximum volume)

So, with this perspective, either digital or analog volume of a 96 db dynamic range CD is more than good enough. Most systems can't even exploit a well recorded CD anyway.

When you get into pro equipment with 120 db SPL continuous output at less than 0.3 THD over the entire frequency range then you can begin to benefit from 24 bit technology (in an audible sense when playing back music)

So from my perspective the benefits of analog over well implemented digital volume control become theoretical in most cases (as you cannot actually hear it in a real room although you might measure it with an oscilloscope)